Steam reforming is a catalytic reaction in which a mixture of steam and gaseous hydrocarbons is exposed to a catalyst at high temperature to produce a mixture of carbon oxides and hydrogen, commonly known as syngas. Syngas may be further converted to a very wide range of bulk and specialty chemicals, including hydrogen, methanol, ammonia, transport fuels and lubricants.
The chemical reactions involved in steam reforming have been well known for many years. Indeed, steam reforming has been used by industry since the 1930s, and steam reforming of natural gas has been the dominant method of hydrogen production since the 1960s, when high pressure operation was introduced.
Two potential problems arising from the reforming reactions include metal dusting and coking, which can lead to process inefficiencies and equipment failure. Metal dusting occurs when the combination of temperature, pressure and composition within a carbonaceous gaseous environment leads to corrosive degradation of alloys into dust. Metal dusting conditions can be difficult to avoid in reformer systems and thus metal dusting is a constant threat. Coking occurs when the gaseous hydrocarbons crack to produce a solid carbonaceous material which may clog or damage flow paths, which can lead to heat transfer and conversion inefficiencies and equipment failure.
Industrial steam reformers are conventionally of tubular construction, employing several large metal tubes packed with the reforming catalyst. The hydrocarbon/steam feed mixture flows through the tubes, contacting the catalyst and undergoing conversion to syngas. Because the reforming reactions are endothermic, heat must be supplied to maintain the required reforming temperatures (generally above 800 C). In conventional tubular reforming systems, this is accomplished by placing the tubes in a combustion furnace, usually fired by natural gas, where the heat is transmitted to the tubes by a combination of convective and radiant heat transfer.
Thus, the successful operation of a tubular reformer relies on maintaining a somewhat delicate balance between the endothermic reforming reactions within the tubes and the heat transfer to the tubes from the furnace combustion. The heat flux through the tube walls must be sufficiently high to maintain the required temperatures for the reforming reactions, but must not be so high as to give rise to excessive metal wall temperatures (accompanied by strength reduction) or to coking of the hydrocarbon at hot spots within the tubes. Therefore, the operation of tubular reformers must be subject to stringent control.
While large-scale tubular reformers have been very successful both technically and economically, small-scale tubular reformers are less successful. Amongst other things, the costs to manufacture, install, maintain and operate tubular reformers on a smaller scale are unattractive.
Smaller users of syngas downstream products such as hydrogen, ammonia and methanol have therefore not found it attractive to establish on-site production facilities for those products. Rather, they generally rely on truck-delivery of cylinders of the product from bulk producers. This solution is becoming less attractive as the price of transport fuels increases. Also, many such users with access to natural gas would prefer to have on-site production facilities not only to avoid transport costs but also to enhance the reliability of their supply. Additionally, much of the world's natural gas supply lies in small fields in remote regions not served by pipelines to the natural gas market. The energy content of this so-called “stranded gas” could be more easily transported to market if the gas were first converted to liquids such as methanol and long-chain hydrocarbons, which may be produced from syngas.
Therefore there is a need for the production of syngas on a smaller scale than has been economically and practically feasible with conventional tubular systems, and that need is likely to increase. There are considerable challenges, however: a smaller-scale system must be reasonably proportionate to large scale plant in initial cost, and operating costs must also be proportionate to the scale of production. Low operating costs require high energy efficiency, minimizing natural gas costs, simplicity of operation and minimizing or avoiding the need for attention from full-time plant operators.
While the amount of heat required by the reforming reactions is fixed by thermodynamics, the overall efficiency of energy usage in the plant is dependent upon the effectiveness with which heat is recovered from the hot syngas and hot combustion flue streams to preheat the cold feeds to reforming temperatures and raise the necessary steam. High-effectiveness feed-effluent heat exchangers and the use of flue-heated pre-reformers can assist in this regard. Importantly, whilst large-scale reforming systems might claim energy efficiency credit for the energy content of excess steam exported to other processes on the site, small-scale reforming systems are unlikely to have an export destination available for excess steam and hence its production does not enhance efficiency.
Both initial capital costs and operational simplicity may be enhanced by minimizing the use of active control, using instead passive control techniques where possible. For example, the suitable splitting of a single stream to pass to several components connected in parallel can be achieved by arranging for suitable relative pressure drops through those components, without the use of control valves. As a further example, the temperature of a stream exiting a heat exchanger can be held within close limits by arranging for the heat exchanger to operate with a small temperature pinch.
An additional consideration in small-scale systems is that the user might not operate continuously at or near full plant capacity, in contrast to large-scale plants. Therefore modulation of throughput through a wide range should be achievable and subject to automation, as should fast start-up and shut-down procedures.
The small-scale reformer should also minimize maintenance requirements.
Thus, there is a need for a small-scale reforming process and apparatus which will accomplish the goal of being capital and operating cost-competitive with large-scale systems as a result of simplicity of control, monitoring and maintenance together with high energy efficiency.